The speed of a general pipeline A/D converter is controlled by amplification time for an operational amplifier to amplify a residual signal and A/D conversion time for the post-stage to perform A/D conversion on the residual signal amplified by the operational amplifier. As a method to shorten the A/D conversion time, it is proposed to perform A/D conversion on upper bits of the residual signal by a first A/D converter before performing A/D conversion on lower bits of the residual signal by a second A/D converter (e.g. a successive approximation A/D converter) utilizing the result of A/D conversion performed by the first A/D converter. This method makes it possible to shorten the A/D conversion time for the successive approximation A/D converter, by which A/D conversion time as a whole can be consequently shortened.
However, in the above conventional method, the residual signal inputted into the first A/D converter and that inputted into the second A/D converter are sampled at the same timing, which causes overhead in the sampling period and imposing a limitation on the speed of A/D conversion.